The mechanics of active clays circulated by salts, acids and bases

A model that accounts for electro-chemo-mechanical couplings in clays, due to the presence of dissolved salts and acids and bases, is developed and applied to simulate experimental data. Chemically sensitive clays are viewed as two-phase multi-species saturated porous media circulated by an electrolyte. To the authors’ best knowledge, no other comprehensive project to embody the effects of pH in the elastic-plastic behavior of geomaterials has been attempted so far.The developments are embedded in the framework of the thermodynamics of multi-phase multi-species porous media. This approach serves to structure the model, and to motivate constitutive equations. The present extension capitalizes upon the earlier developments by Gajo et al. 2002. Electro-chemo-mechanical couplings in saturated porous media: elastic-plastic behaviour of heteroionic expansive clays. Int. J. Solids Struct. 39, 4327-4362] and Gajo and Loret 2004. Transient analysis of ionic replacement in elastic-plastic expansive clays. Int. J. Solids Struct. 41(26), 7493-7531], which were devoted to modeling chemo-mechanical couplings at constant pH.Four transfer mechanisms between the solid and fluid phases are delineated in the model: (1) hydration, (2) ion exchange, (3) acidification, (4) alkalinization. Thus all fundamental exchanges at particle level are fully taken into account. Only mineral dissolution is neglected, since experimental observations indicate a negligible role of mineral dissolution for active clays at room temperature. In particular, the newly considered mechanisms of acidification and alkalinization directly affect the electrical charge of clay particles and thus have a key role in the electro-chemo-mechanical couplings. These four mechanisms are seen as controlling both elastic and elasto-plastic behaviors. Depending on concentrations and ionic affinities to the clay mineral, the transfer mechanisms either compete or cooperate to modify the compressibility and strength of the clay. At given stress, they induce swelling (volume expansion) or shrinking (volume contraction).The framework is sufficiently rich to allow for the simulations of recently performed laboratory experiments on clay samples submitted to intertwined mechanical and chemical loading programmes, involving large modifications in ionic strengths and pH, and leading to significant changes in volume, stiffness and strength.